In conventional far-field optical microscopy, the spatial lateral resolution is fundamentally limited to the order of the wavelength .lambda. of the radiation used. This fundamental diffraction limit is overcome in near-field scanning optical microscopy (NSOM) in which a sample is positioned in the near-field vicinity of a light source of size significantly smaller than .lambda.. Such a microscope is described in EP-A-0 112 401.
A light source substantially smaller than the wavelength of the light used is therefore an essential element of a near-field optical microscope. Moreover, for a correct and satisfactory interpretation of images obtained with a near-field optical microscope, it is necessary to know the size of the sub-wavelength sized optical aperture used as the light source so that the image can be deconvolved from the tip response.
In many near-field optical microscopes currently in use, the light source is produced by a sub-wavelength sized circular aperture at the tip of a metal-coated tapered optical fiber. Such a light source is described for example in EP-A-0 487 233. Visible laser radiation can be ported in the optical fiber down to the aperture vicinity with sufficient efficiency to provide photon throughput in the nanowatt range. Such optical apertures, used as a local light source, have been made with diameters as small as 40 nm. In NSOM, the lateral spatial resolution is no longer limited to the wavelength of the probe light `.lambda.` but is rather of the order of the diameter d=2a of the local source. Subwavelength resolution can be obtained when the object is placed within the depth of field of the local source. For subwavelength sized apertures, this depth of field is of the order of the aperture diameter.
Most present day near-field optical microscopes operate at visible wavelengths i.e. .lambda.=400-1000 nm and have tips with aperture diameters `d` lying in the range of approximately d=50-300 nm. The upper limit to the tip diameter is generally dictated by the fact that the tip diameters need to be significantly lower than .lambda. to warrant the use of a scanning microscope as opposed to a conventional optical microscope. The lower limit to the tip diameter is generally dictated by fabrication problems, since, at the present time, it is difficult to make usable tips with diameters lower than around 50 nm with optical fiber pulling techniques. Therefore the majority of tips currently used have diameters in the range of approximately 1/10.lambda.&lt;d&lt;.lambda..
To establish the size of the aperture, it is usual to perform measurements with a scanning electron microscope (SEM). This is time consuming and requires the use of an SEM, which is a costly piece of capital equipment. Moreover, use of an SEM requires experience and skill. Tips for NSOM are currently sold for prices of the order of $100 whereas SEM characterisation of a tip would cost many times more. An SEM measurement is thus unsuited to a commercial environment. Even in a research environment, many workers active in NSOM do not have the financial and human resources to be able to perform SEM measurements.
Up to now, there has been no simple, quick and reliable way of measuring the optical aperture size of a tip for NSOM.